For the black bear, hibernation is more an adaptation for escaping winter food scarcity than an adaptation for escaping winter cold. Most dens are nearly as cold as the surrounding countryside. Dens may be burrows, caves, hollow trees, or simply nests on the ground. Bears gather leaves, grass, and twigs to make isolative beds on which to curl up, leaving only their well-furred backs and sides exposed to the cold. They sleep alone except for mothers with cubs. Most bears use a different den each year.
Hibernation lasts up to 7 months in the northern regions but is shorter in the South. Bears that find food year-round in the South may not hibernate at all. To survive long winters without eating, drinking, exercising, or passing wastes, hibernating bears cut their metabolic rates in half. Sleeping heart rate drops from a summer rate of between 60 and 90 beats per minute to a hibernating rate between 8 and 40 beats per minute. Rectal temperature drops only slightly, though, from 99-102 degrees F in the summer to 88-98 degrees F during hibernation. Bears can maintain this high body temperature despite their slower metabolism in winter because they develop highly insulative fur and reduce blood supplies to their limbs. Only the head and torso are maintained at the high temperatures. Maintaining the brain at a high temperature enables bears to maintain brain function for tending newborn cubs and responding to danger. Less than 1 percent of black bears die in dens.
Their main threats are flooding and predators (wolves, dogs, active bears, and humans). Bears do not usually die of starvation in dens, most deaths from starvation are before or after hibernation and involve primarily cubs and yearlings. Disease is uncommon. Most parasites of bears are adapted to their host’s hibernation cycle and reduce their demands in winter. Medical researchers are studying black bear hibernation to learn how bears cope with conditions that are problems for people. The findings are aiding studies of human kidney disease, gallstones, obesity, anorexia nervosa, and other problems.
Credit: University of Minnesota – Dr. Lynn Rogers. Photos – Mike McIntosh